Thermal conductivity gauge

ABSTRACT

A thermal conductivity gauge measures gas pressure within a chamber. A sensor wire and a resistor form a circuit coupled between a power input and ground, where the sensor wire extends into the chamber and connects to the resistor via a terminal. A controller adjusts the power input, as a function of a voltage at the terminal and a voltage at the power input, to bring the sensor wire to a target temperature. Based on the adjusted power input, the controller can determine a measure of the gas pressure within the chamber.

BACKGROUND

Because the rate of heat transfer through a gas is a function of the gaspressure, under certain conditions, measurements of heat transfer ratesfrom a heated sensing element to the gas can, with appropriatecalibration, be used to determine the gas pressure. This principle isused in the well-known Pirani gauge, in which heat loss is measured witha Wheatstone bridge network, which serves both to heat the sensingelement and to measure its resistance. In a Pirani gauge, atemperature-sensitive resistance is connected as one arm of a Wheatstonebridge. The temperature-sensitive resistance is exposed to the vacuumenvironment whose pressure is to be measured.

A conventional Pirani gauge is calibrated against several knownpressures to determine a relationship between pressure of a gas and thepower loss to the gas or the bridge voltage. Then, assuming end lossesand radiation losses remain constant, the unknown pressure of a gas maybe directly determined by the power lost to the gas or related to thebridge voltage at bridge balance.

SUMMARY

Example embodiments include a thermal conductivity gauge for measuringgas pressure. The gauge may include a sensor wire, a resistor, and acontroller. The sensor wire may be positioned within a chamber andcoupled to a terminal and a ground. The resistor may be coupled betweenthe terminal and a power input. The controller may be configured toapply the power input to the resistor and adjust the power input, as afunction of a voltage at the terminal and a voltage at the power input,to bring the sensor wire to a target temperature. The controller mayfurther determine a measure of gas pressure within the chamber based onthe adjusted power input.

In further embodiments, the resistor and sensor wire may have anequivalent resistance at the target temperature. The sensor wire may becoupled to a grounded envelope encompassing a volume of the chamber. Thesensor wire may be coupled to the envelope via a shield extendingthrough the volume of the chamber. The controller may be furtherconfigured to 1) determine a compensation factor based on an envelopetemperature external to the chamber, and 2) determine the measure of gaspressure as a function of the compensation factor. The resistor may be afirst resistor, and a second resistor and a switch can be connected inparallel with the first resistor, where the controller selectivelyenables the switch.

In still further embodiments, the gauge may be implemented incombination with an ion gauge (e.g., a hot cathode gauge or a coldcathode gauge) within the chamber. Feedthroughs of the gauge and the iongauge extend through a common feedthrough flange. The gauge occupies asingle feedthrough of the feedthrough flange, where the terminal is thesingle feedthrough. The controller can selectively enable the ion gaugein response to detecting the measure of gas pressure from the thermalconductively gauge below a target threshold. The controller may befurther configured to determine a compensation factor based on heatgenerated by the ion gauge, the controller determining the measure ofgas pressure as a function of the compensation factor. The controllermay selectively disables the ion gauge in response to detecting themeasure of gas pressure from the thermal conductively gauge above atarget threshold.

In yet further embodiments, the sensor wire is supported within aremovable housing extending between the terminal and the ground.

Further embodiments can include a method of measuring gas pressure. Apower input can be applied through a resistor and sensor wire connectedin series, where the sensor wire is coupled to a terminal and a groundwithin a chamber, and the resistor is coupled between the terminal and apower input. The power input can be adjusted, as a function of a voltageat the terminal and a voltage at the power input, to bring the sensorwire to a target temperature. A measure of gas pressure can then bedetermined within the chamber based on the adjusted power input.

Still further embodiments can include a thermal conductivity gauge formeasuring gas pressure, including a circuit and a controller. Thecircuit includes a sensor wire and a resistor coupled in series, thesensor wire being positioned within a chamber. The controller may beconfigured to 1) apply a power input to the circuit; 2) adjust the powerinput, as a function of a voltage across one of the sensor wire and theresistor, to bring the sensor wire to a target temperature; and 3)determine a measure of gas pressure within the chamber based on theadjusted power input.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments, as illustrated in the accompanyingdrawings in which like reference characters refer to the same partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead being placed upon illustrating embodiments.

FIG. 1A is a circuit diagram of a prior art Pirani gauge.

FIG. 1B is a graph illustrating a response of the Pirani gauge of FIG.1A.

FIGS. 2A-B illustrate a prior art Pirani gauge including a compensationwire.

FIG. 3 illustrates the prior art Pirani gauge of FIGS. 2A-B implementedwithin a chamber.

FIG. 4 illustrates a sensor of a thermal conductivity gauge in anexample embodiment.

FIG. 5A illustrates the sensor of FIG. 4 in further detail.

FIG. 5B is a graph illustrating a response of the sensor of FIG. 4.

FIG. 6 is a diagram of a thermal conductivity gauge in an exampleembodiment.

FIGS. 7A-C are graphs illustrating response of the thermal conductivitygauge at different envelope temperatures.

FIGS. 8A-D are plots illustrating response of the gauge implementingtemperature compensation.

FIG. 9A is a diagram of a thermal conductivity gauge in a furtherembodiment.

FIG. 9B is a diagram of a thermal conductivity gauge in a still furtherembodiment.

FIGS. 10A-B illustrate response of a thermal conductivity gauge with andwithout baseline correction.

FIG. 11 illustrates an assembly including a thermal conductivity gaugeimplemented in combination with a hot cathode ion gauge.

FIG. 12 is a diagram illustrating the feedthroughs implemented by theassembly of FIG. 11.

FIGS. 13A-C illustrate a housing supporting a sensor wire for use in athermal conductivity gauge.

DETAILED DESCRIPTION

A description of example embodiments follows.

Pirani sensors with constant sensor wire temperature have been employedto perform pressure measurements between 1E-4 and 1000 Torr. TypicalPirani gauges that provide a constant sensor wire temperature duringoperation rely on a Wheatstone bridge in connection with the sensorwire. The electrical power required to keep the wire at a constanttemperature is used to provide a measure of pressure. Maintaining aconstant temperature at the sensor wire is desirable as it allows fasterresponse to pressure steps as there is no need to wait for temperaturechanges to take place. Also, having constant wire temperature providespressure independent signal baseline offsets that can be subtracted fromthe actual signal to provide the pure pressure dependent part of thesignal by itself.

In a typical constant wire temperature Pirani gauge, the temperature ofa wire is kept at a constant temperature by running pressure dependentelectrical heating power through it. Since the amount of electricalpower needed to keep the wire at a constant temperature depends onpressure, a simple power measurement is used to provide a pressuremeasurement. This design relies on a Wheatstone bridge to regulate wiretemperature by maintaining its temperature dependent resistance duringoperation.

FIG. 1A is a circuit diagram of a prior art Pirani gauge 100. Thepressure sensor comprises a temperature sensitive resistance R_(S)connected as one arm of a Wheatstone bridge 110. R₃ is typically atemperature sensitive resistance designed to have a negligibletemperature rise due to the current i₃. R₂ and R₁ are typically fixedresistances. The sensor wire R_(S) and typically R₃ are exposed to theenvironment whose pressure is to be measured. The environment isencompassed within an envelope through which the sensor wire R_(S)extends via a pair of feedthroughs. Alternatively, R₃ may also beincluded within the envelope via an additional one or more feedthroughs.

The resistance values of resistors R₁, R₂ and R₃ are selected such thatwhen a pressure-dependent voltage V_(Bridge) is applied to the top ofthe bridge, at which V_(left)=V_(right), the resistance of the sensorwire R_(S) is fixed and identical to (R₁*R₃)/R₂. Voltage V_(Bridge) isautomatically controlled by an operational amplifier to maintain thevoltage difference between V_(left) and V_(right) at zero volts. Whenthe potential drop from V_(left) to V_(right) is zero, the bridge isconsidered to be balanced. At bridge balance, the following conditionsexist:i _(s) =i ₃,  (1)i ₁ =i ₂  (2)i _(s) R _(S) =i ₁ R ₁,  (3)i ₂ R ₂ =i ₃ R ₃  (4)Dividing Eq. 3 by Eq. 4 and using Eq. 1 and 2 givesR _(S) =βR ₃  (5)whereβ=R ₁ R ₂  (6)

Thus, at bridge balance, R_(S) is a constant fraction β of R₃.

To achieve a steady-state condition in R_(S) at any given pressure, Eq.7 below must be satisfied:Electrical power input to R _(S)=Power radiated by R _(S)+Power lost outends of R _(S)+Power lost to gas by R _(S)  (7)

Because the amount of electrical power required to keep the sensorresistor R_(S) at a constant temperature and a constant resistanceincreases with pressure, voltage V_(bridge) depends on pressure as well.This relationship is illustrated in FIG. 1B, which is an example plot ofvoltage V_(bridge) over a range of pressure within a chamber occupied byR_(S). As shown, the voltage V_(bridge) exhibits an S-curve over thepressure range. A conventional Pirani gauge is calibrated againstseveral known pressures to determine a relationship between unknownpressure, P_(x), and the power loss to the gas or more conveniently tothe bridge voltage. Then, assuming end losses and radiation lossesremain constant, the unknown pressure of the gas P_(x) may be directlydetermined by the power lost to the gas or related to the bridge voltageat bridge balance.

The Pirani gauge 100 provides a simple configuration for measuringpressure, and allows for adjusting a sensor wire resistance in a simplemanner. A simple op-amp circuit can be used to null the bridge(V_(left)=V_(right)), allowing the circuit to be built at a low cost.However, in order to provide compensation for different ambienttemperatures outside the chamber, resistors of highly specific valuesmust be added to the gauge head during calibration to provide thedesired signal response (i.e., V_(bridge) versus pressure) and propertemperature dependence.

FIGS. 2A-B illustrate a prior art Pirani gauge 200 including acompensation wire Rc. The gauge 200 is comparable to the Pirani gauge100 described above, but the addition of the compensation wire Rc allowsthe gauge 200 to compensate pressure readings against ambienttemperature fluctuations. Such ambient temperature fluctuations changethe difference in temperature between the sensor wire R_(S) and theenvelope walls (not shown) encompassing the chamber in which thepressure is to be measured. As shown in FIG. 2B, the compensation wireresistor R_(C) is wound around a smaller envelope within the chamber andallowed to reach a temperature T₂ having thermal equilibrium with roomtemperature. The resistances in the bridge (R₃ and R₄) and in thecompensation wire Rc are then tuned such that as T₂ changes, and whilethe Wheatstone bridge remains balanced, the difference in temperatureT₁−T₂ (where T₁ is the wire temperature of the sensor R_(S)) remainsconstant. Because the power dissipated by the sensor wire R_(S) to thegas depends on this temperature difference, a measurement of this powerdissipation indicates a pressure measurement that is independent ofambient temperature.

In practice, the compensation wire R_(C) exhibits variability amongdifferent gauges. Thus, each implementation of the gauge 200 must beindividually tuned by adjusting resistance values during testing andcalibration to provide a temperature difference (T₁−T₂) that remainsconstant as the ambient temperature changes. Further, the winding of thecompensation wire R_(C) can be expensive and difficult to complete. Inorder to provide fast response, the compensation wire R_(C) can also bewound internally to the gauge in a thin walled envelope and becomeexposed to the gas environment.

FIG. 3 illustrates the prior art Pirani gauge 200, described above, in afurther view as implemented within a chamber 290 (not shown to scale). Aportion of the gauge 200, including the sensor wire R_(S) and thecompensation wire R_(C), extends into a chamber 290 via a feedthroughflange 220, while the remainder of the Wheatstone bridge remains outsidethe chamber 290. The compensation wire R_(C) is mounted inside thepressure sensor volume on a thin-walled can 240 that facilitatesstabilization of the compensation wire R_(C) while the room temperaturechanges. The gauge 200 requires a minimum of four feedthroughs 210through the feedthrough flange: two feedthroughs connect the sensor wireR_(S), and another two feedthroughs connect the compensation wire R_(C).

The Pirani gauge 200 exhibits several disadvantages. In particular, bothassembly and calibration of the gauge 200 can be difficult andlaborious. In order to assemble and operate the gauge 200, thecompensation wire R_(C) must be wound and attached to electricalconnectors at the feedthrough flange 220. Once assembled, the gauge 200must undergo calibration for proper temperature compensation, includingselecting the proper resistor values and ensuring that the value T₁−T₂remains constant regardless of the room temperature. The Wheatstonebridge requires fine tuning for temperature compensation. Maintainingthe value T₁−T₂ can be achieved if the calibration procedure is properlyexecuted, but it does not allow the use of nominal resistor values.Rather, each gauge must be manually tuned, and is configured withspecific resistors that are high-accuracy components.

Further, the sensor of the gauge 200, including the sensor wire R_(S)and can 240, is large and bulky. In order to achieve convection at highpressures, the can 240 must have a large volume to allow convection toset in as pressure goes above approximately 100 torr. One reason forthis requirement is that the sensor wire R_(S) is not wound or coiled,and the can 240 has a large inner diameter.

A Pirani gauge may be useful a sensor to control enabling and disablingof an ionization gauge (not shown). However, due to its size and use ofmultiple feedthroughs, the gauge 200 may be unsuitable for use incombination with an ionization gauge. An ionization gauge occupiesseveral feedthroughs and substantial volume adjacent to a feedthroughflange, leaving minimal space and feedthroughs for a Pirani gauge.Moreover, temperature compensation is generally required to run a Piranigauge inside the envelope of an ionization gauge. As the ionizationgauge is turned on, the walls of the ionization gauge envelope warm up,making it necessary to add temperature compensation as the differencebetween T₁ and T₂ changes due to an increasing T₂. The use of aninternal compensation wire requires feedthroughs, while the addition ofan external compensation wire adds complexity to the design.

Due to the rigid implementation of temperature control based on aWheatstone bridge, the gauge 200 does not allow for a change of thesensor wire operational temperature (or resistance) during operation,instead providing a single temperature of operation.

Even though there is a linear relationship between pressure and thepower required to keep the sensor wire R_(S) at constant temperature,the gauge 200 indicates pressure based on a measurement of the bridgevoltage V_(bridge), which, as shown in FIG. 1A, is not linearly relatedto pressure. The combination of a large baseline offset (due toradiative and end losses, for example) with a non-linear response ofV_(bridge) on pressure leads to an S-shaped curve that makes calibrationdifficult and less accurate while interpolating the measurement results.

FIG. 4 illustrates a thermal conductivity gauge 400 in an exampleembodiment, with attention to a sensor portion of the gauge. The gauge400 includes a sensor wire R_(S) 405 (also referred to as a filament)fixed within a chamber 490 via a wire mount 406. The wire 405 connectsto the gauge circuit 450 (described in further detail below) via aterminal 412 that extends into the chamber 490 through a singlefeedthrough 410. An opposite node of the wire 405 can be connected to aground, such as an envelope 480 encompassing the chamber 490. Atemperature sensor 470 (e.g., a thermistor) can be positioned at or nearthe envelope 480 to measure temperature of the envelope 480 and/orambient temperature outside the chamber 490.

In contrast with the gauge 200 described above with reference to FIGS.2-3, the gauge 400 provides a sensor having a simpler configuration. Thegauge 200 requires only a single feedthrough 410 into the chamber 490.Further, a compensation wire may be omitted from the gauge 400, astemperature compensation can be provided using the temperature sensor470 in combination with the gauge circuit 450. Thus, the gauge 200enables a sensor having a simpler, more compact structure that requiresless labor to assemble.

The gauge circuit 450 provides further advantages over the gauge 200.The principles on which the gauge circuit 450 operate are describedbelow with reference to FIGS. 5-6.

FIG. 5A illustrates a portion of the gauge 400 in further detail. Here,the sensor is configured with the optional addition of a shield 415. Thesensor wire 405 may be connected between the terminal 412 (embodied as afeedthrough pin) and the shield 415. The shield 415 provides aconductive path to ground, as well as surrounds at least a portion ofthe sensor wire 405, protecting the sensor wire 405 from physical damagefrom contaminants from a process environment and providing a thermalboundary condition for the sensor wire 405. The shield 415, when used incombination with a hot cathode gauge, may also serve to shield thesensor wire from the radiation from the hot filament. In such aconfiguration absent the shield 415, the sensor wire may experience alarge change in the baseline radiation offset. An insulator 411 maysurround the terminal 412 at the feedthrough 410 to ensure a seal withinthe chamber 490. The terminal 412 further connects to the gauge circuit450.

The sensor wire 405 may be a filament of a small diameter (e.g., 0.001in. or 0.002 in) and twisted into a coil (e.g., a coil 0.010 in. indiameter). The operational temperature T1 of the sensor wire 405 can beselected to have a target of 20 C or more above room temperature toprovide adequate sensitivity to pressure changes. The temperature of thesensor wire 405 can be held at a constant value during operation, whichcan improve the speed of response to changes in pressure. This constanttemperature T1 can be achieved by applying a controlled power input(designated P_(W) to distinguish from pressure P) at the terminal 412 tobring the sensor wire 405 toward a target resistance value. A relationbetween the resistance and temperature of the sensor wire 405 can bedetermined for the sensor wire 405 based on previous measurements of thesame wire type. This relation can be used for calibrating the gauge 400.As shown in FIG. 5B, the required power input P_(W) also varies as afunction of the pressure of the chamber 490. This function exhibits alinear region in which the pressure can be measured most accurately.

FIG. 6 is a diagram of the gauge 400 with attention to the gauge circuit405. In view of the relation between the resistance and temperature ofthe sensor wire 405, the gauge circuit 450 can maintain the sensor wire405 at temperature T₁ during operation. To do so for a selectedoperational wire temperature T₁, the corresponding wire resistance canbe calculated based on known calibration curves for the wire type. Theresistance of the sensor wire 405 at the selected temperature is thusR_(S)(T₁).

In order to control the resistance of the sensor wire 405 at differentpressures, the gauge circuit 450 can include a resistor R₁ connected inseries with the sensor wire 405. To simplify analysis, the resistance ofR₁ can be selected to be equal to the resistance of R_(S) at theselected temperature T₁:R ₁ =R _(S)(T ₁)  (8)

A variable voltage source Vh can be connected to the resistor R₁opposite the terminal 412, and the voltage at the terminal Vt and thevoltage source Vh can be compared to determine an adjustment for thevoltage source Vh. In one embodiment, the gauge circuit 450 can providethis comparison and adjustment with an amplifier 452, a comparator 460,and a voltage controller 465. The comparator 460 compares the values of2*Vt (provided by the amplifier 452) and Vh, and outputs a comparisonresult to the voltage controller 465. The voltage controller 465 thenadjusts Vh until 2*Vt is equal to Vh:Vh=2*Vt  (9)

When the above condition is met, the resistance of the sensor wire 405matches the resistance of R₁, and the wire is at temperature T₁. Theelectrical P_(W) power required to heat the sensor wire to temperatureT₁ is then a function of Vh and R₁ as follows:Pw=Vh ² /R ₁ or Pw=4Vt ² /R ₂  (10)

The value P_(W) at this state can be used to calculate pressure based onan observed relationship between pressure and heating power. In exampleembodiments, this relationship can be linear over a pressure rangeextending up to approximately 1 Torr, as illustrated in FIGS. 7A-B,described below. Thus, with the resistor R₁ and sensor wire 405 R_(S)having resistance values selected in accordance with equation (8), thegauge circuit 450 can apply the voltage input Vh to the resistor R₁, andadjust the power input P_(W) as a function of a voltage across theresistor R₁ (i.e., Vh and V_(t)) to satisfy equation (9). In doing so,the sensor wire 405 is brought to the target wire temperature T₁. Ameasure of the power input P_(W) at this state can be determined byequation (10). Comparing the adjusted power input P_(W) against a knownpower/pressure relation, a measure of gas pressure within the chamber490 can thus be determined.

The gauge circuit 450 presents one solution for adjusting thetemperature of the wire to T1, which ensures that the resistance of thesensor wire 405 matches that of resistor R₁ regardless of the gaspressure exposed to the wire. The comparator 460 and voltage controller465 provide a feedback loop to measure the differential between Vh and2*Vt and adjust Vh until the difference is zero and R₁−R_(S)(T₁). Thecomparator 460, amplifier 452 and voltage controller 465, or othercircuitry providing comparable operation, may be implemented in analogand/or digital circuitry.

FIG. 7A is illustrates a response of an example 0.001 in diameter sensorwire implemented in a thermal conductivity gauge such as the gauge 400.The log-log plot shows the change in power response to pressure within achamber as a function of temperature of the envelop housing the chamber.The plot includes four curves, where each curve corresponds to the samewire temperature (100 C) but different envelope temperatures. As shown,the four curves share a common range of pressure (approximately 1E-3Torr to 1 Torr) where the relation between power input and pressure aresubstantially linear.

FIG. 7B illustrates the response of the 0.001 in diameter sensor wire infurther detail. Here, the linear range of the lowest and highestenvelope temperatures (22 C and 54 C) is shown in isolation. This plotdemonstrates how the linear range of the curves is affected by envelopetemperature, where a higher envelope temperature corresponds to a lowerpower input at a given pressure. Both plots can be matched closely witha linear trend estimation as shown. Thus, the power input P_(W) requiredto heat the sensor wire 405 to temperature T1 is a function of thetemperature difference T₁−T₂.

FIG. 7C illustrates a response of an example 0.002 in diameter sensorwire. In contrast to the 0.001 in diameter wire described above, the0.002 in diameter wire exhibits higher power dissipation, therebyrequiring a higher power input. However, the thicker wire also providesa response usable for determining pressure, and may confer advantagesduring installation and operation due to its higher durability. Thesensor wire may be composed of one or more different materials based onthe application and working environment of the gauge. For example, asensor wire made of nickel may be advantageous for use in reactiveenvironments, a tungsten sensor wire may have higher durability, and aplatinum sensor wire may be suitable when lower emissivity is desired.

The results shown in FIGS. 7A-C can be used to provide for envelopetemperature compensation of pressure measurements in a thermalconductivity gauge. For example, with reference to FIG. 4, the sensorwire 405 exhibits a response comparable to the response illustrated inFIGS. 7A-C. The response may be determined based on measuring the sensorwire itself or may be defined from the wire type. A data setcorresponding to this response may be collected and compiled into alookup table cross-referencing power input, pressure, and envelopetemperature. When the gauge 400 operates as described above, theresulting power input P_(W), along with a measure of the enveloptemperature detected by the thermal sensor 470, can be applied to thelookup table to determine the pressure of the chamber 490.Alternatively, the wire response may be used to derive an equationrelating the power input, pressure, and envelope temperature, such as anexpression for the linear trend estimation shown in FIG. 7B. Thus, byapplying data for power input versus pressure across different envelopetemperatures, such as the data shown in FIGS. 7A-C, a measure of powerinput at a gauge can be used to determine chamber pressure in a mannerthat compensates for different temperatures of the envelope. In contrastto the gauge 200 described above, which requires controlling thetemperature difference between ambient and sensor temperatures, thegauge 400 measures the ambient or envelop temperature T₂ independently(e.g., via the thermal sensor 470), and uses this temperature data tointerpret the power measurement when determining pressure.

FIG. 8A illustrates a response of a thermal conductivity gauge with andwithout temperature compensation. Similar to the plots of FIGS. 7A-B,the power response of a given sensor wire at two different envelopetemperatures (25 C and 54 C) is shown. Additionally,temperature-compensated versions of the power response are shown, whichexhibit a nearly identical curve. Thus, as a result of the temperaturecompensation, the power input applied to the sensor wire can be used todetermine the pressure accurately across a range of different envelopetemperatures.

An approach for calculating a temperature-compensated power value is asfollows:

An uncompensated plot of power input and pressure, as shown in FIG. 8B(e.g., based on a direct sensor signal response), can be divided intothree sections 1) baseline at high vacuum (at left), 2) linear response(Log, Log) (at center), and 3) Convection response (˜10 torr to ATM) (atright). The baseline response is made up of two loss mechanism that areconstant over the entire pressure range: 1) R_(L), Radiation loss, and2) C_(L), Convective end loss. Thus, the total baseline loss can beexpressed as:Total baseline loss=R _(L) +C _(L), where

-   -   a) Radiation loss R_(L)=εσ(T⁴ _(wire)−T⁴ _(case)), where        ε=emissivity, σ=Boltzmann's constant    -   b) Convective loss C_(L)=Gπr²(T_(wire)−T_(case))/L, where        G=thermal conductivity of wire, r=radius of wire, L=length of        wire

In the linear response region, L_(R)=KP, (Log L_(R)=log K+P), which isnearly temperature independent (T/sqrt(T)).

In the convection region, hot sheaths of gas inhibit thermal transfer,and the response flattens out. Yet the response also has a Δ T and likeΔ T⁴ dependence and can be modeled. The temperature coefficient of thebaseline loss regions can be corrected with an equation that has thedelta T and delta T to the fourth terms:Delta T Power Baseline=(c+dΔT+eΔT4); where c,d and e can be determinedfrom thermal cycling and fitting population.

The entire sigmoid response function can be modeled as a logistics-typesigmoid function, thereby enabling the device to betemperature-compensated with the known physics of the regions:

-   -   a) Logistics sigmoid function=1/(1+e^(−x)), replacing the e^(−x)        term with the physics of the device.    -   b) Linear region=KP K=constant, P=pressure    -   c) Boundary conditions:        -   i. a=atmospheric power level        -   ii. b=baseline offset bower level

${\left. d \right)\mspace{14mu}{Calculated}\mspace{14mu}{Power}} = {\frac{a}{1 + \frac{1}{KP}} - b}$

Expressing the atmospheric and baseline power levels as a function oftemperature provides the following:a(T)=a(Tnominal)+(c+dΔT+eΔT4)  a)b(T)=b(Tnominal)+(f+gΔT+hΔT4)  b)

${{\left. c \right)\mspace{14mu}{Temperature}\text{-}{compensated}\mspace{14mu}{Calculated}\mspace{14mu}{Power}} = {\frac{a(T)}{1 + \frac{1}{KP}} - {b(T)}}};$

Based on the above equations, an equation to calculate pressure frommeasured power can be expressed as follows:Pressure(T)=(a(T)/K)/(1/(power−b(T))−1)

To calibrate a gauge to provide accurate parameters in the equationabove, the power may be measured at atmosphere and baseline at a nominalpressure. A plot of example temperature-compensated power curves,utilizing the above equations, is shown in FIG. 8A. Here, a pair oftemperature-compensated power curves is compared against a pair ofuncompensated power curves for the same temperatures (25 C and 54 C).FIG. 8C illustrates a comparison between a single power curve,temperature-compensated and uncompensated, within a high-pressureconvection region. Similarly, FIG. 8D illustrates a comparison between asingle power curve, temperature-compensated and uncompensated, within ahigh-vacuum baseline region.

FIG. 9A is a diagram of a thermal conductivity gauge 900 in a furtherembodiment. The gauge 900 may incorporate one or more features of thegauge 400 described above. A circuit 902 includes a transistor 910, aresistor R₁, and a sensor wire 905 (R_(S)) connected in series between avoltage source V_(P) and ground. The circuit 902 extends, in part, intoa chamber 990, where the sensor wire 905 may be configured as describedabove with reference to FIG. 4. A thermal sensor 970 (e.g., athermistor) detects the temperature T₂ of an envelope 980 encompassingthe chamber 980.

A controller 950 may be configured to receive a measure of voltagesV_(H) and V_(T) opposite the resistor R₁, as well as an indication ofthe temperature T₂ from the thermal sensor 970, and outputs a controlsignal V_(C) to control current through the transistor 910. Thecontroller 950 may incorporate features of the gauge circuit 450described above, and may be implemented in analog and/or digitalcircuitry. For example, the controller 950 may include ananalog-to-digital converter (ADC) for converting V_(H), V_(T) and T₂, todigital values; a proportional-integral-derivative controller (PID)controller for determining the control voltage V_(C) based on thedigital values; and a digital-to-analog converter (DAC) for generatingthe control voltage V_(C) to the transistor 910.

Prior to operation, the gauge 900 may be configured comparably to thegauge 400 described above. Further, the resistance value of resistor R₁may be selected based on the room-temperature resistance of the sensorwire 905, where the room-temperature resistance can be used to calculatethe resistance of R₁ required to maintain the operational temperature T₁of the sensor wire, where “tempco” is a temperature coefficient thatindicates the change in resistance with temperature:R ₁ =R _(S)(room temperature)+T ₁*tempco*R _(S)(room temperature)(or R ₁=R _(S)(room temperature)(1+T ₁*tempco)  (11)

In operation, the controller 950 can maintain the sensor wire 905 attemperature T₁ by adjusting the control voltage V_(C), therebycontrolling the power input at the resistor R₁. The controller 950 candetermine adjustment to the control voltage V_(C) through a processcomparable to the process for determining voltage Vh described abovewith reference to FIG. 6. In particular, the voltages VT and VH can becompared to determine an adjustment for the voltage source provided bythe transistor 910. The controller 950 adjusts V_(C) until 2*V_(T) isequal to V_(H). When this condition is met, the resistance of the sensorwire 905 matches the resistance of R₁, and the wire is at temperatureT₁. The electrical power P_(W) required to heat the sensor wire totemperature T₁ is then a function of V_(H) and R₁ as described above inequation (10). The power input P_(W) at this state can be used tocalculate pressure based on an observed relationship between pressureand heating power as illustrated in FIGS. 7A-B and 8. Thus, with theresistor R₁ and sensor wire 905 R_(S) having resistance values selectedin accordance with equation (8), the controller 950 can apply the powerinput P_(W) to the resistor R₁, and adjust the power input P_(W) as afunction of a voltage across the resistor R₁ (i.e., Vh and V_(t)) tosatisfy equation (9). In doing so, the sensor wire 405 is brought to thetarget wire temperature T₁. A measure of the power input P_(W) at thisstate can be determined by equation (10). Comparing the adjusted powerinput P_(W) against a known power/pressure relation, a measure of gaspressure within the chamber 990 can thus be determined. The controller950 may output an indication of P_(W) to enable this determination, ormay be configured to determinate pressure via a lookup table or furthercalculation, thereby outputting a pressure value. In doing so, thecontroller 950 may also use the envelope temperature T₂ to determine atemperature-compensated pressure value as described above.

The controller 950 provides a digital control loop that enables thegauge 900 to be configured to operate with a desired wire temperature ina range of possible temperatures. By changing the multiplication factorbetween Vt and Vh, a target wire temperature can be selected as follows:Vh=xVt where x is a multiplication factor

To derive x:

At Tnominal (room temperature), R1=Rs (R1 connected in series with thesensor wire Rs). At any other temperature, the temperature coefficientof the wire can be used to calculate Rs(T):Rs(T)=(1+α*(Tset−Tnominal))Rs,(where α is the temperature coefficient ofthe wire type to be used)

The relationship between Vh and Vt can be expressed as a simpleresistive divider equation:Vt=[R1/(R1+Rs(T))]VhInserting Rs(T):Vt=[R1/(R1+(1+α*(Tset−Tnominal))Rs)]Vh(using R1=Rs)Vt=[R1/(R1+(1+α*(Tset−Tnominal))R1)]VhVt=[R1/(R1*(1+(1+α*(Tset−Tnominal)))]VhVt=[1/((1+(1+α*(Tset−Tnominal)))]VhVt=[1/((2+α*(Tset−Tnominal))]VhVh=(2+α*(Tset−Tnominal))*VtVh=x*Vtx=(2+α*(Tset−Tnominal))

(2+α*(Tset−Tnominal)) is the multiplication factor (x) in the digitalloop that can be applied to change the temperature of the wire dependingon the customers' requirements and process.

Example values that may be implemented in the calculations above are asfollow:

α=0.0048 (TC for tungsten)

Tset=100, Tnominal=75

Vh=2.36*Vt; x=2.36

Tset=125 Tnominal=25

Vh=2.48*Vt; x=2.48

Thus, using a calculated multiplication factor x, the wire temperaturecan be configured for a given application of the gauge 900 withoutchanging the values of resistors R₁ or R_(S).

FIG. 9B is a diagram of a thermal conductivity gauge 901 in a furtherembodiment. The gauge 901 may include the features of the gauge 900 ofFIG. 9A, with the addition of a resistor R₂ that can be selectivelyconnected in parallel with the resistor R₁ via a switch 934. The switchmay be controlled by the controller 950 or may be manually configured.In alternative embodiments, one or more additional resistors may beselectively connected in series with, or in parallel with, the resistorR₁. The resistor R₁ may also be replaced with a variable resistor.

Further, the temperature of the wire can be adjusted by using a variablemultiplication factor. For example, R1=Rs may be set at ambienttemperature. When the wire is at the desired temperature (Vh=2*Vt), themultiplication factor x can be used to adjust the temperature such thatVh=x*Vt, where x=(2+tempco*(Ttarget−Tambient). Under such animplementation, only the ambient resistances of the sensor (Rs) must bematched to R1.

In some applications, an adjustable resistance provided by the gauge 901may be advantageous. For example, the gauge 901 may be used in multiplesettings requiring different operational temperatures of the sensor wire905. FIG. 10A illustrates one such application, where the power responseof the sensor wire 905 over pressure is shown for two differentoperational temperatures T₁ (100 C and 120 C) of the sensor wire 905.The sensor wire 905 exhibits a lower power response at 100 C than at 120C. To correct for this difference, as shown for example in FIG. 10B, thevalues of resistors R1 and R2 may be selected such that (1) the resistorR1, absent R2, exhibits a target response from the sensor wire 905 whenT1 is at a first operational temperature (e.g., 100 C); and (2) thetotal resistance of R1 and R2 connected in parallel exhibits acomparable response from the sensor wire 905 when T1 is at a secondoperational temperature (e.g., 120 C). As a result, the gauge 901 canprovide for operation at different values of T1 while achieving acomparable response for determining pressure.

FIG. 11 illustrates an assembly 1100 including a thermal conductivitygauge 1120 implemented in combination with an ion gauge 1130. The iongauge 1130 includes an electron source 1105, an anode 1120, and an ioncollector electrode 1110. The electron source 1105 (e.g., a hot cathode)is located outside of an ionization space or anode volume. The anodestructure includes a cylindrical wire grid 120 around posts 112 and 114,defining the ionization space in which electrons 1125 impact gasmolecules and atoms. The ion collector electrode 1110 is disposed withinthe anode volume. Electrons travel from the electron source 1105 to andthrough the anode, cycle back and forth through the anode 1120, and areconsequently retained within, or nearby to, the anode 1120. Furtherembodiments may utilize an ion gauge having a cold cathode electronsource.

In their travel, the electrons 1125 collide with molecules and atoms ofgas that constitute the atmosphere whose pressure is desired to bemeasured. This contact between the electrons and the gas creates ions.The ions are attracted to the ion collector electrode 1110, which isconnected to an ammeter 1135 to detect current from the electrode 1110.Based on a measurement by an ammeter 1135, the pressure of the gaswithin the atmosphere can be calculated from ion and electron currentsby the formula P=(1/S) (I_(ion)/I_(electron)), where S is a coefficientwith the units of 1/Torr and is characteristic of a particular gaugegeometry, electrical parameters, and pressure range.

The gauge 1120 may be configured as described above with reference toFIGS. 4-10. Due to the low pressures at which a typical ion gaugeoperates, the assembly 1100 benefits from the thermal conductivity gauge1120, which can measure higher pressures than the ion gauge 1130.Further, the thermal conductivity gauge 1120, via a controller 1150, maycontrol a switch for the ion gauge 1130, enabling the ion gauge 1130(e.g., at power sources 1113, 1114) when the measured pressure fallsbelow a given threshold, and disabling the gauge 1130 when the measurepressure rises above a given threshold. As a result, the ion gauge 1130can be prevented from operating at pressures that may cause damage toit. Conversely, the controller 1150 may receive input from the ion gauge1130 (e.g., from ammeter 1135), enabling the gauge 1120 when thepressure rises above a threshold and disabling the gauge 1120 when thepressure falls below a threshold.

In response to the heat generated by the ion gauge 1130 duringoperation, the thermal conductivity gauge 1120 may be further configuredto compensate for temperature fluctuations caused by this heat. Forexample, to the extent that the ion gauge 1120 raises the temperature ofthe envelope, the thermal conductivity gauge 1120 may compensate forthis temperature change as described above with reference to FIGS. 7A-Cand 8. This approach may also be applied to temperature changes measuredat other points relative to the ion gauge 1130. For example, a thermalsensor may be implemented at the ion gauge 1130 to measure temperature,and this measured temperature can be correlated to the power response ofthe thermal conductivity gauge 1120 to determine a compensation factorbased on heat generated by the ion gauge, thereby enabling the gauge1120 to determine the measure of gas pressure as a function of thecompensation factor.

When implemented in combination, the thermal conductivity gauge 1120 andion gauge 1130 may be assembled such that feedthroughs of each gaugeextend through a common feedthrough flange 1145. An example feedthroughflange 1145 is illustrated in a top-down view, in FIG. 12. The ion gauge1130 uses several feedthroughs of the flange 1145. In contrast, thethermal conductivity gauge 1120, as configured as described above,requires only a single feedthrough 1160. This single feedthrough 1160accommodates a feedthrough pin, and the sensor wire R_(S) is connectedbetween the feedthrough pin and a ground (e.g., the envelope or a post).Because the gauge 1120 only requires a single feedthrough of the flange1145, and an ion gauge may leave at least one feedthrough unused in anexisting assembly, the gauge 1120 may provide a further benefit in thatit can be retrofitted into such an existing assembly with minimalalteration to the assembly.

FIGS. 13A-C illustrate a housing 1300 supporting a sensor wire 1305 foruse in a thermal conductivity gauge such as the gauges 400, 900described above. FIG. 13A illustrates a side cross-section of thehousing 1300, and FIG. 13B is an perspective view of the housing 1300.The housing 1300 may include conductive end caps 1320 to which thesensor wire 1305 can be connected. The sensor wire may be retained ateach end by being compressed between the end caps and a tube 1350,thereby making the electrical connection. The tube 1350 can be of anon-conductive material (e.g., glass or ceramic), and connects the endcaps 1320. Depending on a desired level of thermal transfer between thesensor wire 1305 and the chamber, the tube 1350 may be closed orslotted.

FIG. 13C illustrates a top-down view of the housing 1330 as positionedwithin a chamber. The housing 1300 provides a number of advantages. Forexample, the rigid structure of the tube 1350 protects the wire fromdamage during installation and operation. Further, the end caps 1320 mayaccommodate a bracket or post within the chamber, allowing the housing1300 to be quickly and easily installed, removed and replaced.

While example embodiments have been particularly shown and described, itwill be understood by those skilled in the art that various changes inform and details may be made therein without departing from the scope ofthe embodiments encompassed by the appended claims.

What is claimed is:
 1. A thermal conductivity gauge for measuring gaspressure, comprising: a sensor wire within a chamber coupled to aterminal and a ground; a resistor coupled between the terminal and anelectrical input receiving a power input; a controller configured to 1)apply the power input to the resistor; 2) adjust the power input, as afunction of a voltage at the terminal and a voltage at the electricalinput, to bring the sensor wire to a target temperature; and 3)determine a measure of gas pressure within the chamber based on anadjusted power input at the target temperature.
 2. The gauge of claim 1,wherein the resistor and the sensor wire have an equivalent resistanceat the target temperature.
 3. The gauge of claim 1, wherein the sensorwire is coupled to a grounded envelope encompassing a volume of thechamber.
 4. The gauge of claim 3, wherein the sensor wire is coupled tothe envelope via a shield extending through the volume of the chamber.5. The gauge of claim 1, wherein the controller is further configuredto 1) determine a compensation factor based on an envelope temperatureexternal to the chamber and 2) determine the measure of gas pressure asa function of the compensation factor.
 6. The gauge of claim 1, whereinthe resistor is a first resistor, and further comprising a secondresistor and a switch connected in parallel with the first resistor, thecontroller configured to selectively enable the switch.
 7. The gauge ofclaim 1, further comprising an ion gauge within the chamber.
 8. Thegauge of claim 7, wherein feedthroughs of the thermal conductivity gaugeand the ion gauge extend through a common feedthrough flange coupled tothe chamber.
 9. The gauge of claim 8, wherein the thermal conductivitygauge occupies a single feedthrough of the feedthrough flange, theterminal being the single feedthrough.
 10. The gauge of claim 7, whereinthe controller is configured to selectively enable the ion gauge inresponse to detecting the measure of gas pressure being below a targetthreshold.
 11. The gauge of claim 7, wherein the controller is furtherconfigured to determine a compensation factor based on heat generated bythe ion gauge, the controller configured to determine the measure of gaspressure as a function of the compensation factor.
 12. The gauge ofclaim 7, wherein the controller is configured to selectively disable theion gauge in response to detecting the measure of gas pressure beingabove a target threshold.
 13. The gauge of claim 1, wherein the sensorwire is supported within a removable housing extending between theterminal and the ground.
 14. A method of measuring gas pressure,comprising: applying a power input through a resistor and sensor wireconnected in series, the sensor wire being coupled to a terminal and aground within a chamber, the resistor being coupled between the terminaland an electrical input receiving a power input; adjusting the powerinput, as a function of a voltage at the terminal and a voltage at theelectrical input, to bring the sensor wire to a target temperature; anddetermining a measure of gas pressure within the chamber based on anadjusted power input at the target temperature.
 15. The method of claim14, wherein the resistor and the sensor wire have an equivalentresistance at the target temperature.
 16. The method of claim 14,wherein the sensor wire is coupled to a grounded envelope encompassing avolume of the chamber.
 17. The method of claim 16, wherein the sensorwire is coupled to the envelope via a shield extending through thevolume of the chamber.
 18. The method of claim 14, further comprising 1)determining a compensation factor based on an envelope temperatureexternal to the chamber, and 2) determining the measure of gas pressureas a function of the compensation factor.
 19. The method of claim 14,wherein the resistor is a first resistor, and further comprisingselectively connecting a second resistor in parallel to the firstresistor based on the voltage at the terminal.
 20. The method of claim14, further comprising selectively enabling an ion gauge in response todetecting the measure of gas pressure being below a target threshold.21. The method of claim 14, further comprising determining acompensation factor based on heat generated by an ion gauge, the measureof gas pressure being determined as a function of the compensationfactor.
 22. The method of claim 14, wherein the sensor wire is supportedwithin a removable housing extending between the terminal and theground.
 23. A thermal conductivity gauge for measuring gas pressure,comprising: a circuit comprising a sensor wire and a resistor coupled inseries, the sensor wire being positioned within a chamber; and acontroller configured to 1) apply a power input to the circuit; 2)adjust the power input, as a function of a voltage across one of thesensor wire and the resistor, to bring the sensor wire to a targettemperature; and 3) determine a measure of gas pressure within thechamber based on an adjusted power input at the target temperature.